I have the same problem, too with my Samsung Galaxy A8. The earphone jack always works normally when I connect my earphones or earphones/mic. However, it never works correctly when I connect an 3.5 mm aux cable directly from the earphone jack to the aux-in on my sound system. It doesn't switch to send the sound through the aux cable, and keeps playing it out the phone's speaker. SOLUTION THAT WORKED FOR ME: I found an un-elegant work around... I have added a line splitter (one of those that you can use for sharing your music with a friend when you both are using earphones). I plug in a set of earphones to one side of the splitter, then use an aux cord to go from the other side of the splitter to the aux-in on my sound system, and it works. There is some volume loss due to splitting the signal, so the volume on my phone and on my sound system have to be turned up higher than normal, but the phone switches its output properly, and sends music through the aux cable to the sound system (and to the earphones).